Determination of voltage dependence of capacitance of 100 kV and 300 kV compressed gas capacitors using the kinetic method

Measuring voltage dependence capacitance of high voltage compressed gas capacitors is challenging when levels are below one ppm (one Micro-Farad per Farad). Indeed, traditional methods such as direct capacitor comparison, voltage transforming, voltage-doubling, direct voltage, frequency doubling, and simplified tilting are not suitable because they need complex procedures, reference standards with known voltage dependence and accurate capacitance bridges. The kinetic method could be used to determine a possible quasi-zero voltage dependence or at least to detect it. A 100 kV capacitor was studied using this technique at LNE with support from PTB. An excellent initial eccentricity, equal to (0,070 ± 0,010) mm, was determined and which is in accordance with what is claimed by the manufacturer. Quasi-zero voltage dependence (0.45 ± 0.10) ppm, was determined. This low level of uncertainty is difficult to reach with any other method, the internal dimensions of the capacitor have to be estimated. Another 300 kV capacitor was also investigated without any knowledge of its internal dimensions. It was concluded that it has a medium eccentricity despite its good calibration results, equal to (7.0 ± 5.0) ppm.